Question 5 of 7: Emission Source Characterization and Gaseous Pollutant Control

Question 5: Emission Source Characterization and Gaseous Pollutant Control (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Point, Line, Area and Volume Source Characterization

A point source (a single stack or vent with well-defined location, height and exit velocity) is appropriate whenever emissions genuinely originate from one identifiable release point, such as a power-plant or industrial stack — the Gaussian plume equation in Question 2 is derived explicitly for this case. A line source is appropriate for emissions distributed continuously along a linear path with a roughly constant emission rate per unit length, the classic example being a roadway (mobile-source traffic emissions) or a pipeline; it is modelled as an integral (or superposition) of point sources along the line. An area source is used when emissions arise from many small, individually-insignificant, spatially distributed release points over a region that cannot practically be resolved as separate points — residential heating, small commercial solvent use, or a tank farm's aggregate fugitive losses across a facility footprint. A volume source is appropriate for a source with a genuine initial three-dimensional dimension at release — a building rooftop monitor, a fugitive source inside a large open structure, or an already well-mixed plume close to a source cluster — where treating the release as a point would understate the near-field concentration because the true initial spread is not negligible.

(ii) NO_x Formation in Natural-Gas Combustion and Post-Combustion Control

Natural gas is essentially sulphur- and nitrogen-free as a fuel, so fuel-bound NO_x is negligible; NO_x instead forms almost entirely as thermal NO_x via the Zeldovich mechanism, in which atmospheric N₂ and O₂ dissociate and recombine as NO at the very high flame temperatures (>1600–1800 °C) reached in the primary combustion zone; formation rate rises exponentially with peak flame temperature and increases with excess-air (available O₂) and residence time at peak temperature. A smaller prompt-NO_x contribution also forms via hydrocarbon radicals reacting with N₂ early in fuel-rich flame zones. An effective post-combustion technology is selective catalytic reduction (SCR): ammonia (or urea, hydrolyzed to ammonia) is injected into the flue gas upstream of a catalyst bed (typically vanadium/titanium-oxide based), where it reacts selectively with NO_x over NO/NO₂ at 300–400 °C, $$4\text{NO}+4\text{NH}_3+\text{O}_2\rightarrow 4\text{N}_2+6\text{H}_2\text{O},$$ achieving 80–90%+ NO_x removal without generating a secondary waste stream.

(iii) Opacity Monitoring for Particulate Control

Opacity is the fraction of a fixed light beam obscured by particulate matter in the stack gas as it crosses the stack diameter, measured continuously by a transmissometer (light source and photodetector on opposite sides of the duct, or a folded-path unit). Because opacity correlates strongly with particulate mass loading for a given particle size distribution and colour, a continuous opacity monitor (COM) gives operators and regulators a real-time, low-cost surrogate for particulate emission rate without needing a continuous mass measurement (which is far harder to automate). Regulatory opacity limits (e.g., a 20% six-minute average ceiling, the historical EPA Method 9 style limit mirrored in Canadian provincial air regulations) let a facility demonstrate continuous compliance and immediately flag control-equipment upsets — a sudden opacity spike signals a baghouse bag failure or ESP trip long before a periodic stack test would catch it, allowing corrective action (bypass, alarm, load reduction) before a large uncontrolled particulate release occurs.